The present invention relates specifically to the display of the image of a scanning electron microscope system. The invention provides among its objectives, an improvement of resolution available upon a TV-like display of the specimen image painted upon a cathode ray tube.
Much of the theoretical and experimental work with scanning electron microscopes was undertaken in the 1930's. This body of instrumentation utilized a thermionic source for electrons (heated filament) which produced an electron beam which was focused and accelerated and further caused to illuminate a specimen. One of the inherent characteristics of the thermionic systems is a low current beam which when used in the high magnification ranges for which electron microscopy is particularly useful, required repeated scans and recording of an image upon photographic film. This was due to the insufficiency of beam current and existent technology to provide a suitable signal/detector link to provide "real time" viewing as by a TV-type screen.
The advent in the late 1960's of field emission sources in electron microscopy provided a beam brightness which found an immediate adaptability to TV-type television image signal processing. As field emission microscopy developed into a commercial reality, a need developed within that industry for dynamic, flicker-free TV viewing, high resolution and stereoscopic TV systems.
The normal television system in the United States is based on a 525 line scanning system having a two-to-one interlace ratio. This means that two fields of 2621/2 lines are placed on the screen, the second field of lines being interposed between the lines of the first. While 525 lines have been found quite adequate for normal viewing of TV, at the distances usually encountered, the photographing of such screens and the images thereon renders such systems unsatisfactory. It has been found that the enlargement of the photographic images produced by scanning electron microscopes reveals the spaces between successive scans and thus generates lined photographs.
A logical improvement to the elimination of such lines would be to increase the number of scanning lines placed upon the face of the tube. However, such an improvement is not without substantial drawbacks. Those skilled in TV systems will immediately recognize the higher numbers of lines enormously expands the required bandwidth of the broadcast (and display) system and thus significantly increases costs and complexity.
It will be further recognized by those skilled in the art that bandwidth requirements can be reduced by a technique of alternative line scanning called interlacing. This philosophy is well known in the art and described in several references, such as Millman and Taub's "Pulse and Digital Circuits" McGraw Hill, 1956 pp. 515-517. The domestic TV industry by way of example, uses a 2:1 interlace, as mentioned above. Extensions of the philosophy beyond this basic 2:1 interlace system are not without substantial problems, however. Many of these are elaborated upon in U.S. Pat. No. 3,767,926 (RE 28,153), issued to Vincent J. Coates and Leonard M. Welter and commonly assigned to American Optical Corporation, the assignee of the present application. The display system disclosed therein is directed to overcoming the problems of extended interlace. The invention provided a system with an interlace of greater than 2 to 1 and included nonsequential interlacing to avoid a cascade or waterfall effect which was discovered and identified in the development of that invention.
The present invention provides, among its objects high numbered interlace patterns to improve the resolution available for line-free photomicrographs in scanning electron microscopy. The invention also provides a changeability of interlace among certain preselected patterns, to adapt the interlace pattern to the needs of the photomicrograph and the anticipated enlargement thereof.
It should be understood, however, that the application of the invention is not restricted to the field of scanning electron microscopy. TV (cathode ray tube) displays are in common usage in a variety of information systems, and the present invention may be adapted to use therein wherever a need for multiple interlace is advantageous.